Various technologies require the heating of material to achieve a transition of the material from an initial state to a final state exhibiting desired characteristics. For example, heat is employed to recover polymeric heat recoverable articles such as heat shrink tubing and molded parts, cure gels, melt or cure adhesives, activate foaming agents, dry inks, cure ceramics, initiate polymerization, initiate or speed up catalytic reactions, or heat treat parts among other applications.
The speed at which the material is heated is a significant consideration in the efficiency and effectiveness of the overall process. In ultraviolet, infrared, hot air, hot liquid, and flame heating methods, or other examples where external heat sources are used, it is often difficult to obtain uniform heat distribution in the material through to its center. In instances where the center of the material is not adequately heated, its transition from the initial state may not fully or uniformly occur. Alternatively, in order to obtain the desired temperature at the center of the article, excessive heat may be required to be applied at the surface whereby such excessive temperature conditions can lead to degradation of the material surface. Additionally, the extended time required to apply heat to accomplish the transition to the desired state diminishes the cost-effectiveness of the system. In cases where thermally conductive fillers are used in the material to improve the heat transfer from the surface of the material to its center, large amounts of filler that may adversely affect the properties of the host material are necessary for a smooth temperature gradient.
Because of these disadvantages of external heating, bulk, or internal heating methods are preferred to provide fast, uniform, and efficient heating.
In contrast to the external heating methods discussed above, electromagnetic heating techniques, such as microwave, dielectric and magnetic induction, all provide internal heating of non-conductive articles, such as polymeric heat recoverable articles, gels, adhesives, foams, inks and ceramics. The electromagnetic energy is indirectly coupled to the material and heat is generated uniformly within the bulk of the material.
Both microwave and dielectric heating techniques are based primarily on the heat generated in the dielectric material by the "rattling" of electric dipoles as they try to align with a rapidly alternating applied electric field. Microwave heating requires exposure to fields at frequencies in the high Megahertz or Gigahertz range where water dipoles resonate. The presence of water on the surface of a dielectric material to be heated with microwaves may result in non-uniform heating of the material. Dielectric heating employs frequencies from about 27 MHz to high Megahertz where the electric dipoles of most dielectrics resonate. The dielectric material being heated in this fashion does not have an inherent temperature control; the oscillating electric dipoles continue to generate heat, thereby causing degradation of the material when the heating is excessive.
Magnetic induction heating employs alternating magnetic fields such as those produced in an induction coil to couple with a work piece situated inside the coil. A magnetic or electrically conductive material can couple with the applied field and thereby transform the coupled electromagnetic energy into thermal energy. A non-magnetic and electrically non-conductive material is transparent to the magnetic field and therefore cannot couple with the field to generate heat. However, such a material may be heated by magnetic induction heating by uniformly distributing ferromagnetic particles within the material and exposing the article to an alternating high frequency electromagnetic field. Small sized ferromagnetic particles are efficient heat generators when exposed to alternating fields of frequency from about 100 kHz to about 50 MHz.
Materials suitable for induction heating include ferromagnetic and ferromagnetic materials. In this application, we use the definition of ferromagnetic and ferrimagnetic materials as set forth in a publication by R. M. Bozorth entitled "Ferromagnetism", Bell Telephone Laboratories, Inc. D. Van Nostrand Company, Inc., 1951, which is hereby incorporated by reference for all purposes. Ferrimagnetic materials, or ferrites, are a subgroup of ferromagnetic materials. A detailed analysis of ferrimagnetism is set forth by Smit and Wijn in "Ferrites", John Wiley & Son. 1959, which is hereby incorporated by reference for all purposes. Ferrimagnetic materials usually exhibit very low electrical conductivity compared to ferromagnetic metals and metal alloys.
Ferromagnetic materials such as iron, nickel, cobalt, iron alloys, nickel alloys, cobalt alloys, permalloy, and several steels, and ferrimagnetic materials such as magnetite, nickel-zinc ferrite, manganese-zinc ferrite, and copper-zinc ferrite are all suitable as heat generating particles dispersed in a non-magnetic, electrically non-conductive host material exposed to a high frequency alternating magnetic field. Though electrically conductive, non-magnetic metals such as copper, aluminum and brass may be used in the form of particles to produce heat, they are less efficient than magnetic materials and are therefore not preferred.
Ferromagnetic materials generate heat primarily due to combination of induced eddy currents and magnetic hysteresis losses.
Alternating magnetic fields induce eddy currents in particles comprising electrically conductive material. These internally circulating currents can produce heat within a particle. The majority of the induced eddy currents are confined within a distance .delta. from the surface of the particle given by the formula: EQU .delta.=(2/.omega..sigma..mu..sub.r).sup.1/2
where .sigma. is the electrical conductivity of the particle in ohm.sup.-1 -m.sup.-1, .omega. is the angular frequency of the applied field in sec.sup.-1, and .mu..sub.r is the magnetic permeability of the particle relative to air. This distance .delta. is defined as the particle "skin depth" when the particle is exposed to an alternating magnetic field. At a distance .delta. the current density has dropped to 1/e, or about 37% of its value at the surface. Therefore, a particle comprising a ferromagnetic material of electrical conductivity .sigma. and relative magnetic permeability .mu..sub.r exposed to an alternating electromagnetic field of frequency .omega., has a skin depth defined by the above equation.
Electrically conductive ferromagnetic particles of a size several times larger than the particle skin depth may be efficient generators of heat from eddy currents. Small skin depth may be achieved with particles of high magnetic permeability and high electrical conductivity exposed to a magnetic field of high frequency. For example, nickel with an electrical conductivity of 1.3.times.10.sup.7 ohm.sup.-1 -m.sup.-1, a relative permeability of 100 (.mu..sub.r =100.times.4.pi..times.10.sup.-7 Wb/A-m) exposed to a field of frequency 5 MHz (.omega.=2.pi.f=2.pi..times.5.times.10.sup.6 sec.sup.-1) gives a skin depth of 6.2 .mu.m. Thus, about 37% of the induced current density will be confined in a region of the particle 6.2 .mu.m from the surface of the particle. The magnitude of the induced current density increases with the size of the eddy current loop and hence with the size of the particle.
Eddy current losses are negligible in electrically less conductive particles due to the large skin depth of such particles. For example, a manganese-zinc ferrite, such as ferrite Mn-67 from Ceramic Magnetics, with an electrical conductivity of 0.67 l ohm.sup.-1 -m.sup.-1, and a relative magnetic permeability of 4000 exposed to a field of frequency 5 MHz has a skin depth of 435 .mu.m and particles greater than about a millimeter are necessary for the generation of eddy current losses. Such large particles will adversely alter the properties of the host material and are, hence, undesirable. Similarly, an electrically non-conductive nickel-zinc ferrite, such as CMD 5005 from Ceramic Magnetics, with an electrical conductivity of 1.0.times.10.sup.-7 ohm.sup.-1 -m.sup.-1, a relative permeability of 3000 exposed to a field of frequency 5 MHz has a skin depth of 1.3.times.10.sup.7 .mu.m or 13 m.
Electrically non-conductive ferrimagnetic particles such as ferrite particles, or electrically conductive ferromagnetic particles that have all three dimensions smaller than the skin depth heat up primarily due to magnetic hysteresis losses. The magnetic dipoles within each magnetic domain of the particle tend to align with the rapidly alternating magnetic field thereby resulting in domain wall movement. If the alignment of the dipoles is not in phase with the field, the alignment lags the field and follows a hysteresis loop. The hysteresis loop represents the response of the ferromagnetic material to an applied magnetic field and its size and shape depend on the properties of the ferromagnetic material and on the strength of the applied field. The area enclosed by the hysteresis loop represents the work required to take the material through the hysteresis cycle. When this cycle is repeated, dissipative processes within the material due to realignment of the magnetic domains result in a transformation of the magnetic energy into internal thermal energy which raises the temperature of the material. Hysteresis losses do not depend on the particle size as long as the particle size is equal to at least one magnetic domain.
The amount of heat generated by particles dispersed in an electrically non-conductive, non-magnetic host material depends on several parameters including the following equipment and particle parameters:
Equipment parameters:
Coil size and geometry PA1 Coil current frequency PA1 Coil current amplitude (power) PA1 Coil efficiency PA1 Magnetic permeability PA1 Electrical conductivity PA1 Size and shape of hysteresis loop PA1 Particle volume fraction in the host material PA1 Geometry PA1 Size PA1 Alignment with the field and with each other PA1 Proximity to coil PA1 said first and second orthogonal dimensions are greater than the skin depth of the particle; and PA1 said first and second orthogonal dimensions are at least 5 times said third orthogonal dimension. PA1 a heat activatable blocking construction positioned in proximity to the wires of the cable, said blocking construction comprising a host material in which particles are dispersed, said particles comprising ferromagnetic material having high magnetic permeability and high electrical conductivity, said particles having a skin depth and a configuration including first, second and third orthogonal dimensions, wherein said first and second orthogonal dimensions are greater than the skin depth of the particle and said first and second orthogonal dimensions are at least about 5 times said third orthogonal dimension; and PA1 a cover disposed around said blocking construction.
Particle parameters:
For a given frequency, power, and coil size and geometry, faster heating of the host material containing the ferromagnetic particles may be obtained by carefully selecting the particle properties. Particles of the present invention are highly efficient in that they provide fast heating at low particle volume fractions in the host material, thereby having no adverse effect on the host material properties.
When a magnetic particle reaches or exceeds a critical temperature, referred to as the Curie temperature, or Curie point, its magnetic permeability drops precipitously to a value approaching 1. The particle then loses much of its ability to respond to a magnetic field and heating is significantly diminished. When the temperature of the particle drops below the Curie point, the particle regains its magnetic properties and heating resumes. Therefore, when the temperature of the particle is less than the Curie point, the particle heats. When the temperature of the particle is greater than the Curie point, the particle essentially stops increasing in temperature. Therefore, the particle autoregulates. Thus, the Curie point is a practical autoregulation means for preventing the host material from being overheated.
It is known to intersperse particles in a polymeric material which are heated by induction. Examples can be found in U.S. Pat. Nos. 3,620,875; 3,391,846; 3,551,223; 3,620,876; 3,709,775; 3,902,940; 3,941,6411. 4,000,760; 4,918,754; and 5,123,989; 5,126,521; PCT International Publication WO 90/03090, Defensive Publication T905,001 published Dec. 19, 1972 by E. I. du Pont de Nemours and Company; Japanese Patent Applications S(56) (1981)--55474; S64 (1989) 4331; and H3 (1991)--45683; and Swedish Patent Specification 224,547, which are all hereby incorporated by reference for all purposes.
Attempts have been made to provide regulation of the temperature of the host material by selecting particles with a Curie point equal to or slightly greater than the temperature to which the article is to be heated. Examples include U.S. Pat. Nos. 2,393,541; 3,551,223; 4,555,422; 4,699,743 and 5,126,521, and PCT International Publication WO 91/11082, which are all hereby incorporated by reference for all purposes.
Uniform dispersion of the particles throughout the bulk of the material facilitates uniform heating. In this way, induction heating also allows selective and controlled heating. Selective heating can result where the particles are placed in higher concentrations in areas to be heated to a relatively greater extent. Additionally, the temperature of articles loaded with ferromagnetic particles and heated by induction heating may be controlled by utilization of particles having a Curie point near the desired temperature.